Two-dimensional metal oxide nanosheets for rechargeable batteries

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Review

Two-dimensional metal oxide nanosheets for rechargeable batteries Jun Mei, Ting Liao, Ziqi Sun∗

Q1

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, 2 George Street, Brisbane, QLD 4000, Australia

a r t i c l e

i n f o

Article history: Received 24 September 2017 Revised 7 October 2017 Accepted 12 October 2017 Available online xxx Keywords: 2D nanomaterials Metal oxide Lithium-ion battery Rechargeable batteries Sodium-ion battery

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a b s t r a c t Two-dimensional (2D) metal oxide nanosheets have attracted much attention as potential electrode materials for rechargeable batteries in recent years. This is primarily due to their natural abundance, environmental compatibility, and low cost as well as good electrochemical properties. Despite the fact that most metal oxides possess low conductivity, the introduction of some conductive heterogeneous components, such as nano-carbon, carbon nanotubes (CNTs), and graphene, to form metal oxide-based hybrids, can effectively overcome this drawback. In this mini review, we will summarize the recent advances of three typical 2D metal oxide nanomaterials, namely, binary metal oxides, ternary metal oxides, and hybrid metal oxides, which are used for the electrochemical applications of next-generation rechargeable batteries, mainly for lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs). Hence, this review intends to functionalize as a good reference for the further research on 2D nanomaterials and the further development of energy-storage devices. © 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Jun Mei received his Bachelor’s degree (2013) and Master’s degree (2016) in chemical engineering and technology from Shandong Normal University and from Changchun University of Technology (China), respectively. He is currently pursuing his Ph.D. degree at the School of Chemistry, Physics and Mechanical Engineering (CPME) of Queensland University of Technology (QUT) under the supervision of Dr. Ziqi Sun. His major research interests include the design and synthesis of novel nanomaterials for energy conversion and storage devices.

∗

Corresponding author. E-mail address: [email protected] (Z. Sun).

Dr. Ting Liao received her Ph.D. degree from the Institute of Metal Research, Chinese Academy of Sciences (China) in 2009. After finishing her postdoctoral fellowship at the National Institute for Materials Science (NIMS, Japan), which she held for 1 year, she joined the University of Queensland (UQ, 2010–2013) and then the University of Wollongong (UOW, 2014–2016) with funding from a UQ fellowship and a UOW-VC fellowship, respectively. Ting Liao is now a Senior Lecturer at Queensland University of Technology (QUT, 2017-present) and a 2016 ARC Future Fellow recipient. She is also serving as an editorial board member of Scientific Reports (Nature Group). Her major research interest is the first-principles computation of nanomaterials in energy applications.

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Dr Ziqi Sun received his Ph.D. degree from the Institute of Metal Research, Chinese Academy of Sciences in 2009. After finishing his NIMS postdoctoral fellowship (Japan) that he held for 1 year on solid oxide fuel cells, he joined the University of Wollongong (UOW) with funding from an ARC-APD fellowship (2010), a UOW-VC fellowship (2013), and an ARC-DECRA fellowship (2015). Ziqi is currently a senior lecturer at the Queensland University of Technology (QUT). He is also serving as the Editor of Sustainable Materials and Technology (Elsevier), Associate Editor of Surface Innovation (ICE Publishing), and Editorial Board Members of Scientific Reports (Nature Group) and the Journal of Materials Science and Technology (Elsevier). His major research interests include metal oxide nanomaterials and bio-inspired inorganic nanomaterials for sustainable energy harvesting, conversion, and storage.

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https://doi.org/10.1016/j.jechem.2017.10.012 2095-4956/© 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Please cite this article as: J. Mei et al., Two-dimensional metal oxide nanosheets for rechargeable batteries, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.10.012

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1. Introduction With the purpose to address the tough challenge originating from the global climate change and the limited fossil fuels on earth, sustainable energy-related technologies are being persistently pursued [1–7]. Among them, energy storage systems are essential as the uninterrupted power supply sources. As one of primary energy-based storage systems, rechargeable batteries have been extensively applied for electric vehicles (EVs) and portable electronic devices [8–12], which is primarily due to their remarkable capacity, attractive specific energy density, outstanding cycling stability, and low self-discharge as well as low memory effect [13,14]. These rechargeable batteries, however, still need further improvement in order to meet the increasing demands for long-life service, lighter weight, and higher safety [15,16]. Practically, most of these concerns are in connection with the electrode materials, which significantly influence the overall battery performances. To date, a wide range of electrode materials have been studied, their electrochemical performances, however, are still far from satisfaction for practical applications [17–21]. Recently, the development of graphene nanosheets has ushered two-dimensional (2D) nanomaterials into the limelight for energy conversion and storage devices [22–28]. These graphenelike nanostructures, including metal oxide nanosheets, transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), graphitic carbon nitride (g-C3 N4 ), MXene, etc., feature atomiclevel thickness, large surface area, tunable electronic properties, remarkable mechanical strength, and unique confined effect [29–35]. Among them, the low-cost 2D metal oxide nanosheets possess many distinctive characteristics for electrochemical reactions, such as ample active sites resulting from the high surface area and superior reaction kinetics contributing by the shortdistance transport pathway. Particularly, as the thickness of 2D nanosheets reduced into a few unit-cell layers, some physical and chemical properties (e.g. bandgap, wettability, in-plane transport, etc.) will become distinct from their bulks or rigid and thick nanosheets, and these changes may affect their electrochemical properties for ion transport and storage [2,26,36–38]. Up to now, a variety of metal oxide nanosheets (e.g. V2 O5 , MnO2 , SnO2 , Co3 O4 , Fe2 O3 , etc.) have been successfully fabricated and explored as cathode/anode materials for different types of batteries, including lithium-ion batteries (LIBs), sodium-ion batteries (NIBs), metal–sulfur batteries, metal–air/oxygen batteries, and so on [26,36,39,40]. Most studies on using metal oxide nanosheets as electrode materials focus on LIBs and SIBs. Furthermore, metal oxide nanosheets are good candidates for metal–air/oxygen batteries owing to their highly catalytic activity toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) [41–44]. In addition, these metal oxide nanosheets can be employed as efficient sulfur hosts for metal–sulfur batteries to effectively promote the redox activities and reduce the dissolution of polysulfides into electrolytes [45–47]. Meanwhile, in the emerging magnesiumion batteries (MIBs), some layered metal oxides, such as MnO2 [48,49] and V2 O5 [50], have been reported as promising candidate as cathode materials. In this review, three typical types of 2D metal oxide nanomaterials, e.g., binary metal oxides, ternary metal oxides, and hybrid metal oxides, for rechargeable batteries are summarized, as the schematic illustration shown in Fig. 1. The 2D binary metal oxides refer to the oxides consisting of only one metal element in the formula, such as Co3 O4 , V2 O5 , MnO2 , Fe2 O3 , SnO2 , NiO, etc., while the 2D ternary metal oxides represent the three-element oxides with two metal elements in the composition, such as LiCoO2 , NiCo2 O4 , NaMnO2, etc. Generally, the simple binary metal oxides are one class of the most widely studied electrode materials, but they often possess relatively low conductivity. As a re-

sult, they often suffer obvious volume expansion/contraction during the repeated ion insertion/deinsertion processes, which further leads to the serious agglomeration and even crack or pulverization of the active materials. Compared with the simple binary metal oxides, the ternary metal oxides with one more metal cation can alloy more active ions such as Li+ and Na+ and often manifest a higher conductivity originating from the lower activation energy for electron transfer. The hybrid metal oxides are composed of one 2D metal oxide and a proper-complementary material (e.g. nanocarbon, carbon nanotubes (CNTs), graphene, etc.). These hybrid multifunctional composites have various configurations, such as particles-on-sheet (0D–2D), wire-on-sheet (1D–2D), sheeton-sheet (2D–2D), and 3D multiscale structures assembled from low-dimensional constituents, which usually exhibit unique synergistic effects owing to a combination of the merits of each individual component [51–54]. The introduction of conductive heterogeneous materials can effectively overcome the intrinsic issues of the low conductivity and the large volume change of individual metal oxides, making these 2D metal oxides more attractive for electrochemical applications [55–57].

107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124

2. Fabrication of 2D metal oxide nanomaterials

125

The fabrication of well-controlled 2D nanomaterials is always a grand challenge. To date, many excellent review articles have been published on the fabrication of 2D nanostructures [60–66]. In summary, the fabrication methods for 2D metal oxide nanosheets are classified as two main categories, namely, “top-down” and “bottom-up” routes (Fig. 2a). The former is often achieved by the exfoliation of their corresponding layered host crystals into metal oxide nanosheets under thermal treatment or in some organic solutions. This is a simple method and can produce high-crystallinity products in a large scale. Zhao et al. reported the massive production of large quantities of ultrathin binary metal oxide nanosheets (e.g. Cr2 O3 , ZrO2 , Al2 O3 , Y2 O3 , etc.) by directly heating the corresponding metal chloride precursors (e.g. CrCl3 ·6H2 O, ZrOCl2 ·8H2 O, AlCl3 ·6H2 O, YCl3 ·6H2 O, etc.) [67]. The effective exfoliation of this method relies on a rapid evaporation of the water vapor and/or other gas molecules generated from the decomposition of precursor salts during thermal treatment process [46]. In this study, the as-obtained Cr2 O3 nanosheets exhibited stronger adhesion to copper foils than the referenced particles without the use of binder and presented enhanced electrochemical performances [67]. Some drawbacks, however, are still remaining in this method, such as that products are not uniform and this strategy is unsuitable for non-layered metal oxide precursors. Further study on the synthesis parameters to improve the quality of 2D nanomaterials is still requested. The “bottom-up” route, including physical/chemical vapor deposition (PVD/CVD) and wet-chemical synthesis, is another common fabrication strategy for various metal oxide nanosheets [60]. Wet-chemical synthesis, for example, is a dominant technology to produce 2D metal oxide nanomaterials as electrode materials for batteries. As summarized in Table 1, most of these reactions are conducted through a hydrothermal/solvothermal process. The bottom-up synthesized metal oxide nanosheets usually possess a thickness in a few nanometers and a large specific surface area. This approach is advantageous for the massive production of uniform products in high-yield. Sun et al. proposed a generalized molecular self-assembly approach for the first time to achieve the controllable synthesis of ultrathin 2D nanosheets of transition metal oxides (TMOs), including TiO2 , ZnO, Co3 O4 , WO3 , Fe3 O4 , MnO2 , etc. [30]. In this synthetic procedure, the inverse lamellar reverse micelles, formed by a co-surfactants system composed of the amphiphilic block copolymers (P123) and short-chain alcohol (ethylene glycol), play a crucial role for the confined growth of

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Fig. 1. Overview of 2D metal oxide nanomaterials for batteries. (Left) Schematic illustration of the configuration of rechargeable Li-ion batteries (LIBs). Reprinted from Ref. [58]. (Right) Schematic illustration of a hybrid anode of lithium–sulfur batteries. Reprinted from Ref. [59].

Fig. 2. (a) Schematic illustration of the fabrication strategies of 2D binary metal oxide nanomaterials. (b) Transmission electron microscopy (TEM) image and colloidal acetone dispersion (inset) of V2 O5 nanosheets, (top) obtained via an exfoliation of layered bulk material method (bottom). Reprinted from Ref. [71]. (c) TEM image and Tyndall phenomenon (inset) of VO2 (B) nanosheets (top) fabricated via an intercalation–deintercalation method (bottom). Reprinted from Ref. [72]. (d) TEM image of porous Co3 O4 nanofoils (top) fabricated via a graphene-mimicking method (bottom). Reprinted from Ref. [73]. (e) TEM image of V2 O5 nanosheets (top) synthesized via a freezedrying method (bottom). Reprinted from Ref. [74].

Please cite this article as: J. Mei et al., Two-dimensional metal oxide nanosheets for rechargeable batteries, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.10.012

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Battery type

LIBs

Materials

Ultrathin V2 O5

Synthesis method

Liquid exfoliation

Morphology and structure

Potential windows

Coulombic efficiency

Initial capacity

Rate capability

Cycling stability

4.0–2.05 V

∼99% (>1st cycle)

4.0–2.0 V

—

4.0–2.5 V

∼100%

3.0–1 V

—

2.0–0.01 V

∼43.7% (1st)

292 mAh g−1 (59 mA g−1 ) 303 mAh g−1 (50 mA g−1 ) 141 mAh g−1 (100 mA g−1 ) 164.8 mAh g−1 (4 C) 1742 mAh g−1 (156 mA g−1 ) 2260.7 mAh g−1 (200 mA g−1 ) 2226 mAh g−1 (100 mA g−1 ) 1007.4 mAh g−1 (89 mA g−1 ) 1574.7 mAh g−1 (200 mA g−1 ) 1643 mAh g−1 (50 mA g−1 ) 216 mAh g−1 (20 mA g−1 ) 214 mAh g−1 (50 mA g−1 ) 1239.7 mAh g−1 (500 mA g−1 ) 1023 mAh g−1 (100 mA g−1 ) 1072 mAh g−1 (100 mA g−1 )

117 mAh g−1 (14.7 A g−1 ) 104 mAh g−1 (5.0 A g−1 ) 106 mAh g−1 (5.0 A g−1 ) 111.8 mAh g−1 (20 C) —

274 mAh g−1 (50th cycles at 58.8 mA g−1 ) 206 mAh g−1 (100th cycles at 0.5 A g−1 ) 92.6% of the 1st capacity (500th cycles at 1.5 A g−1 ) 120.2 mAh g−1 (200th cycles at 10 C) 534 mAh g−1 (50th cycles at 156 mA g−1 ) 757.6 mAh g−1 (40th cycles at 200 mA g−1 ) 1300 mAh g−1 (100th cycles at 1.0 A g−1 ) 1279.2 mAh g−1 (50th cycles at 89 mA g−1 ) 715.2 mAh g−1 (130th cycles at 0.2 A g−1 ) 900 mAh g−1 (200th cycles at 1.0 A g−1 ) 73% of the 1st capacity (100th cycles at 0.1 A g−1 ) 108 mAh g−1 (50th cycles at 0.5 A g−1 ) 172.9 mAh g−1 (100th cycles at 0.5 A g−1 ) 266 mAh g−1 (100th cycles at 1.0 A g−1 ) 452 mAh g−1 (10 0 0th cycles at 1.0 A g−1 )

b

SIBs

a b c

Leaf-like V2 O5

Ultrasonic treatment

Ultra-large V2 O5

Solvothermal

TiO2

Hydrothermal

SnO2

Hydrothermal

SnO2

Microwave-assisted

Fe2 O3

2.0–0 V

∼51.1% (1st)

Galvanic replacement

T: ∼2.1 nm SSA: ∼180.3 m2 g−1 T: ∼2–3 nm SSA: ∼135.7 m2 g−1 T: ∼9.5 nm

3.0–0.01 V

∼72% (1st)

Co3 O4 nanofoils

Graphene template

SSA: ∼48.1 m2 g−1

3.0–0.01 V

∼67.0% (1st)

NiO

Microwave-assisted

T: ∼1.4 nm

3.0–0 V

∼89.9% (1st)

MnO2

Growth on Ni foam

T: ∼20–30 nm

3.0–0 V

—

Sponge-like V2 O5

Hydrothermal

4.0–1.25 V

—

VO2

Hydrothermal

T: ∼15 nm SSA: ∼41.9 m2 g−1 S: ∼50-60 in diameter

4.0–1.5 V

—

Co3 O4

Self-assembly

2.5–0.01 V

∼46.8% (1st)

NiO

Solvothermal

3.0–0.005 V

∼37.7% (1st)

SnO on carbon cloth

Hydrothermal

2.5–0.005 V

∼79.1% (1st)

T: ∼1.5 nm SSA: ∼156 m2 g−1 T: ∼4–5 nm T: ∼1–3 nm SSA: ∼163.6 m2 g−1

Ref.

c

464 mAh g−1 (1.5 A g−1 ) >800 mAh g−1 (10 A g−1 ) 650 mAh g−1 (4.45 A g−1 ) 287.1 mAh g−1 (1.5 A g−1 ) ∼700 mAh g−1 (1.0 A g−1 ) 46 mAh g−1 (0.6 A g−1 ) 70 mAh g−1 (1.0 A g−1 ) 116.9 mAh g−1 (5.0 A g−1 ) 154 mAh g−1 (10 A g−1 ) 410 mAh g−1 (2.0 A g−1 )

[71] [100] [101] [102] [103] [104] [105] [73] [106] [107] [74] [108] [39]

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T: ∼2.1–3.8 nm SSA: ∼147.5 m2 g−1 T: ∼60–80 nm SSA: ∼28 m2 g−1 T: ∼2–5 nm S: > 100 μm T: ∼10 nm a

Electrochemical performance

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[109] [110]

T refers to the thickness of 2D nanosheets. SSA refers to the calculated largest specific surface area. the potential versus Li+ /Li for LIBs and Na+ /Na for SIB.

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Table 1. Summary of synthesis methods, structural parameters, and electrochemical properties of 2D binary metal oxide nanomaterials.

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metal oxides along the thickness dimension. The as-obtained ultrathin metal oxides sheets possessed a thickness in the order of a few nanometers, corresponding to 2–7 stacking layers of monolayer (or unit cell). Moreover, the specific surface area between 150 and 300 m2 g−1 was achieved for the obtained TMO nanosheets. Later, salt-templated wet-chemical synthesis was reported for the massive preparation of 2D TMOs, such as h-MoO3 , MoO2 , MnO, h-WO3 , etc. [68]. The planar growth of these nanosheets was hypothesized to occur via a match between the growing oxides and the crystal lattices of the salts. The major features of this method include its simplicity, low cost, and the easy removal of salts template. More recently, graphene oxide (GO) has been used as sacrificial templates to promote the 2D planar growth of metal oxides onto sheet-like structures [69,70]. Peng et al. reported the template-assisted fabrication of holey TMO nanosheets with adjustable hole sizes [70]. This approach can be used to synthesize both some binary metal oxides (e.g. Fe2 O3 , Co3 O4 , Mn2 O3, etc.) and some ternary metal oxides (e.g. ZnMn2 O4 , ZnCo2 O4 , NiCo2 O4, CoFe2 O4 , etc.). Furthermore, these holey metal oxide nanosheets exhibited strong mechanical stability inherited from the GO templates and presented very small structural changes during the lithiation/delithiation processes as electrode materials for LIBs [70].

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3. 2D binary metal oxide nanomaterials for LIBs and NIBs

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2D binary metal oxide nanosheets encompass a broad selection of different metal cations involving many transition metal elements (e.g. Co, Ti, Ni, V, Cr, Fe, Mn, Cu, etc.) and some main group elements (e.g. Sn, In, Bi, etc.) in the periodic table. Fig. 2(b–e) shows some representative 2D metal oxide nanomaterials and their fabrication methods [71–74]. For example, as demonstrated in Fig. 2(b), few-layer V2 O5 nanosheets with a thickness of approximately 2.1– 3.8 nm have been successfully synthesized by the liquid-exfoliation of layered V2 O5 bulk material in formamide solvent [71]. In respect of the storage mechanisms for Li+ and Na+ , these metal oxides can be categorized into three types, e.g., insertiontype (MOx + yLi+ /Na+ + ye− ↔ Liy MOx /Nay MOx ), alloying-type (Mx Oy + 2y Li+ /Na+ + 2y e− ↔ xM + yLi2 O/Na2 O; M + zLi+ /Na+ + z e− ↔ Liz M/ Naz M), and conversion-type (Mx Oy + 2y Li+ /Na+ + 2ye− ↔ xM + yLi2 O/Na2 O). Table 1 compares the electrochemical performances of some representative 2D metal oxide nanosheets for LIBs and NIBs. For Li+ storage, the insertion-type and alloyingtype electrode materials possess relatively narrow potential windows (∼2.0 V) compared with the conversion-type materials (e.g. Fe2 O3 , Co3 O4 , NiO, MnO2 , etc.), and the later usually have potential windows of approximately 3.0 V. Meanwhile, the alloying-type materials (e.g. SnO2 , etc.) present higher initial capacities of over 1700 mAh g−1 than these of the insertion-type materials (∼100– 300 mAh g−1 ) and the conversion-type materials (∼1500 mAh g−1 ). Nevertheless, the alloying-type materials have lower initial Coulombic efficiency (∼50 %) with serious capacity fading after only 50 cycles caused by the obvious volume changes during cycles. When these metal oxide nanosheets are investigated as electrode materials for NIBs, not only the cycling stability but also the rate capability are unsatisfactory, especially the insertion-type materials (e.g. V2 O5 , VO2 , etc.), which usually have a potential window of about 2.5 V and a reversible capacity ranging from 100 to 400 mAh g−1 . For example, Co3 O4 , as a typical conversion-type electrode materials for LIBs, exhibits a theoretical capacity as high as 890 mAh g−1 based on the reaction mechanism from Co3 O4 to a mixture of metallic cobalt and Li2 O (Co3 O4 + 8Li+ + 8e− ↔ 3Co + 4Li2 O). Moreover, the practical specific capacity of Co3 O4 may become higher. Eom et al. synthesized the porous 2D Co3 O4 nanofoils by using GO as a sacrificial template (Fig. 2d), and these nanofoils delivered a discharge capacity of 1279.2 mAh g−1 (89 mA g−1 ) af-

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ter 50 cycles as electrode materials for LIBs [75]. Besides, Dou et al. prepared the atomically thin Co3 O4 nanosheets grown on the stainless steel mesh and used as an anode material for SIBs, which delivered an average capacity of 509.2 mAh g−1 (50 mA g−1 ) for the initial 20 cycles and an average capacity of 427.0 mAh g−1 at a high rate of 500 mA g−1 [39].

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4. 2D ternary metal oxide nanomaterials for LIBs and NIBs

239

Compared with binary metal oxides, ternary metal oxides have attracted much attention as electrode materials for LIBs and NIBs. It is noteworthy that the synthesis of ternary metal oxide nanosheets, however, is much difficult. Hu et al. reported a rapid mass production of 2D ion-intercalated metal oxide and hydroxide nanomaterials, including Li2 WO4 , Na2 W4 O13 , Na0.55 Mn2 O4 •1.5H2 O, K0.27 MnO2 •0.54H2 O, Zn5 (OH)8 (NO3 )2 •2H2 O, Cu2 (OH)3 NO3 , etc., via a molten salts method (MSM), as shown in Fig. 3(a) [76]. Compared with conventional MSMs, the feature of this method is the direct use of naked ionized ions without hydration in the molten state to guide the planer growth of nanostructures within 1 min. The as-obtained product exhibited an atomic-scale thickness. For example, the 2D Na2 W4 O13 nanosheets presented a thickness of around 3.79 nm, corresponding to about 9 monolayers [76]. There are two metal cations in ternary metal oxides, which can generate an attractive synergistic effect, due to the different expansion coefficients and oxidation states of the cations. Furthermore, the combination of two or more cations can lead to improved electrical conductivity and high Li+ storage capacity [14]. As demonstrated in Table 2, most ternary metal oxide nanosheets show a high initial capacity of over 1200 mAh g−1 and a high firstcycle Coulombic efficiency of more than 80%. These ternary metal oxide materials can be divided into two main categories: Li/Narich layered metal oxides (e.g. LiCoO2 , LiNiO2 , LiMn2 O4 , NaTiO2 , NaCoO2, NaMnO2 , etc.) [77–80], and other ternary metal oxide materials (e.g. NiCo2 O4 , Fe2 (MoO4 )3 , etc.) [81,82]. In the first category, spinel Li4 Ti5 O12 (LTO) is one of the most promising candidates as anode materials for LIBs due to its inherent advantages. First, the operating voltage (1.55 V) is relatively high, avoiding the formation of solid-electrolyte interphase (SEI) layers and inhibiting the electrolyte reduction reactions. Second, this zero-strain insertion material exhibits remarkable insertion/extraction reversibility, thermal stability, superior safety, and environmental benignity. The fabrication of LTO into 2D dimensionality for batteries has the potential to further amplify the advantages of LTO material. Xiao et al. reported the fabrication of LTO nanosheets (ca. 3.2 nm in thickness), which were stacked by ultrathin nanoflakes from the exfoliation of layered orthorhombic precursors (Li1.81 H0.19 Ti2 O5 •xH2 O) (Fig. 3b) [83]. When applied as anode materials for Li+ storage, these nanosheets presented an initial discharge capacity of 175.9 mAh g−1 and the cycling stability was appealing with the capacity of 166.8 mAh g−1 remained after 100 cycles at 0.5 C) [83]. Self-supported LTO nanosheets grown directly on conductive titanium foil were hydrothermally synthesized with the presence of LiOH and employed as binder-free anode for LIBs to avoid the use of extra additives (e.g. carbon black, polymeric binders, etc.) as those in a traditional electrode fabrication process [84]. Electrochemical tests indicated that this anode material exhibited an attractive cycling performance with a capacity retention of about 124 mAh g−1 after 30 0 0 charging/discharging cycles at 50 C. Furthermore, this binder-free anode could be used to assemble a flexible full battery (Fig. 3c), which could be fully recharged to 3.0 V under 30 s and a red LED was able to be lighted when the battery was in bended state (Fig. 3d) [84]. Similar design with aligned nanosheet arrays on conductive substrates have also been reported with a different combination of nanosheets and sub-

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Battery type

LIBs

Materials

Li4 Ti5 O12

Solvothermal and calcination

Mesoporous Li4 Ti5 O12

Hydrothermal and calcination

b c d

T: ∼3.2 nm b SSA: ∼15.5 m2 g−1 SSA: ∼150.6 m2 g−1 a

T: ∼14 nm

NiCo2 O4

Microwave-assisted

SSA: ∼143 m g

NiCo2 O4

Microwave-assisted

NiCo2 O4 on Ni foam

Solution reaction and annealing

NiMoO4

Electrochemical and calcination

T: ∼1.3 nm SSA: ∼126.46 m2 g−1 T: ∼10 nm SSA: ∼112.87 m2 g−1 Honeycomb-like

ZnCo2 O4

Microwave-assisted

ZnCo2 O4

Hydrothermal

Hexagonal Co2 GeO4

Hydrothermal

CoFe2 O4

Thermal decomposition

CoMoO4 on CFc

Hydrothermal

Crumpled ZnMn2 O4

Electrospinning

NiCo2 O4

Hydrothermal

Li4 Ti5 O12

Hydrothermal

2

−1

T: ∼1.2 nm SSA: ∼181.78 m2 g−1 T: ∼2.4–4.3 nm SSA: ∼52.7335 m2 g−1 T: <10 nm SSA: ∼77 m2 g−1 T: ∼30–60 nm SSA: ∼ 12.13 m2 g−1 T: ∼3–5 nm SSA: ∼ 100.26 m2 g−1 T: ∼10–20 nm T: <10 nm SSA: ∼119 m2 g−1 T: ∼10–30 nm

Electrochemical performance d

Potential windows

Coulombic efficiency

2.5-1.0 V

∼100 %

2.5-1.0 V

—

2.5-1.0 V

—

3.0-0.01 V

—

3.0-0.01 V

∼80 % (1st)

3.0-0.01 V

∼71.6 % (1st)

3.0-0.01 V

∼62.7 % (1st)

3.0-0 V

∼82.9 % (1st)

3.0-0.001 V

∼94.3 % (3rd)

3.0-0.01 V

∼78 % (1st)

3.0-0.005 V

∼70.4 % (1st)

3.0-0.005 V

∼83.46 % (1st)

3.0-0.01 V

∼99 %

2.5-0.01 V

∼100% (>5th)

2.5-0.3 V

—

Ref. Initial capacity

Rate capability

Cycling stability

175.9 mAh g−1 (0.5 C) 169 mAh g−1 (1 C) 163 mAh g−1 (20 C) 798 mAh g−1 (100 mA g−1 ) 1287.1 mAh g−1 (200 mA g−1 ) 1738 mAh g−1 (200 mA g−1 ) 1688 mAh g−1 (200 mA g−1 ) 1310.9 mAh g−1 (200 mA g−1 ) 1251.8 mAh g−1 (1.0 A g−1 ) 1275 mAh g−1 (220 mA g−1 ) 1619 mAh g−1 (50 mA g−1 ) 1376.5 mAh g−1 (100 mA g−1 ) 1626 mAh g−1 (100 mA g−1 ) 1143.5 mAh g−1 (100 mA g−1 ) 145 mAh g−1 (1 C)

100.2 mAh g−1 (20 C) 140 mAh g−1 (30 C) 78 mAh g−1 (200 C) 487 mAh g−1 (1.0 A g−1 ) 283.8 mAh g−1 (2.0 A g−1 ) 398.2 mAh g−1 (4.0 A g−1 ) 370 mAh g−1 (8.0 A g−1 ) 372.1 mAh g−1 (2.0 A g−1 ) 410 mAh g−1 (5.0 A g−1 ) ∼250 mAh g−1 (6.95 A g−1 ) 303 mAh g−1 (10 A g−1 ) 604 mAh g−1 (3.2 A g−1 ) 277 mAh g−1 (1.0 A g−1 ) 141.8 mAh g−1 (1.0 A g−1 ) 67 mAh g−1 (40 C)

166.8 mAh g−1 (100th cycles at 0.5 C) — −1

124 mAh g (30 0 0th cycles at 50 C) 767 mAh g−1 (50th cycles at 0.1 A g−1 ) 804.8 mAh g−1 (100th cycles at 0.2 A g−1 ) 1170.1 mAh g−1 (50th cycles at 0.2 A g−1 ) 680 mAh g−1 (200th cycles at 1.0 A g−1 ) 87.9% of the 1st capacity (200th cycles at 0.2 A g−1 ) 810 mAh g−1 (200th cycles at 1.0 A g−1 ) 1026 mAh g−1 (150th cycles at 0.22 A g−1 ) 806 mAh g−1 (200th cycles at 1.0 A g−1 ) 990.02 mAh g−1 (150th cycles at 0.1 A g−1 ) 461 mAh g−1 (500th cycles at 0.1 A g−1 ) 203.7 mAh g−1 (50th cycles at 0.2 A g−1 ) 91% of the 1st capacity (400th cycles at 1C)

[83] [111] [84] [112] [81] [113] [85] [114] [115] [116] [117] [118] [119]

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Morphology and structure

Hydrothermal

Li4 Ti5 O12 on Ti foil

SIBs

Synthesis method

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[120] [121]

T refers to the thickness of 2D nanosheets. SSA refers to the calculated largest specific surface area. full name of the abbreviation of CF is carbon fabric. the potential versus Li+ /Li for LIBs and Na+ /Na for SIB.

[m5G;October 28, 2017;14:8]

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(2017), https://doi.org/10.1016/j.jechem.2017.10.012

Table 2. Summary of synthesis methods, structural parameters, and electrochemical properties of 2D ternary metal oxide nanomaterials.

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Fig. 3. (a) Schematic illustration of the synthesis approach of 2D ternary metal oxide and hydroxide nanomaterials. Reprinted from Ref. [76]. (b) Schematic illustration of the mechanisms of Li4 Ti5 O12 nanosheets stacked by ultrathin nanoflakes via a phase transition process. Reprinted from Ref. [83]. (c and d) Optical photographs of a fabricated flexible battery (c) with a red LED lightened by this battery under bending (d). Reprinted from Ref. [84]. (e) Schematic illustration of the advantages of well-aligned NiMoO4 nanosheets on Ni foam substrate. Reprinted from Ref. [85]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

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strates, such as NiMoO4 nanosheet vertically grown on Ni foam (Fig. 3e) [85]. This nanosheet-on-conductive substrate architecture has many structural advantages for electrochemical reactions, including ample open spaces, high surface area, and robust adhesion. As a result, the ion diffusion and electron transport can be greatly enhanced and the serious volume changes during the repeated insertion/deinsertion behaviors can also be effectively inhibited. Even though the appealing advantages of 2D ternary metal oxide nanosheets, the overall electrochemical performances of LTO nanosheets as electrode materials for SIBs are still poor and further effort are requested to make.

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5. 2D hybrid metal oxide nanomaterials for LIBs and NIBs

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2D hybrid metal oxide nanostructures are usually synthesized via a mixed “top-down” and “bottom-up” method or a two-step self-assembly method, where a 2D nanomaterial that serves as templates or substrates were first fabricated via a “top-down” or a “bottom-up” method and the secondary 2D or other dimensional nanomaterial grows on the templates/substrates [86]. For instance, Yang et al. demonstrated the fabrication of sandwich-like mesoporous titania (G-TiO2 ) nanosheets with template-assisted

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method [87]. During the synthetic procedure, GO-silica template was prepared though the hydrolysis of silica source on the GO surfaces. After thermal treatment under inert condition, graphenesilica nanosheets were obtained [65]. Subsequently, graphene-silica nanosheets were used as a template for the following growth of TiO2 nanosheets with (NH4 )2 TiF6 as precursor, and the sandwichlike G-TiO2 nanosheets were produced after the removal of silica in alkali solutions [87]. When used as a Li+ anode material, they showed excellent rate capability and good cycle performance [87]. This hybridization of 2D metal oxides with some propercomplementary materials into multifunctional heterostructures or composites, including 0D–2D, 1D–2D, 2D–2D, and 3D multiscale structures assembled from low-dimensional constituent nanostructures, is a common and effective approach to further enhance the electrochemical performances of 2D binary and ternary metal oxide nanomaterials. As shown in Fig. 4(a), various nanostructures with different dimensionality, such as 0D nanoparticles/quantum dots (QDs), 1D nanowires/nanorods, 2D nanosheets, etc., have been developed to hybridize with 2D metal oxide nanosheets to form hybrid architectures. The hybridization can effectively improve the overall electronic conductivity, especially for these composites hybridizing with some conductive matrix, such as nano-carbon,

Please cite this article as: J. Mei et al., Two-dimensional metal oxide nanosheets for rechargeable batteries, Journal of Energy Chemistry (2017), https://doi.org/10.1016/j.jechem.2017.10.012

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Fig. 4. (a) Schematic illustration of the configurations of 2D hybrid metal oxide nanomaterials. (b) Schematic illustration of the synthesis of atomically thin mesoporous Co3 O4 nanosheets/graphene composite (ATMCNs-GE); (c) the comparison of cycling performances and Coulombic efficiencies of TMCNs-GE, Co3 O4 nanosheets (ATMCNs), and graphene at the rate of 2.25 C. (b, c) Reprinted from Ref. [90]. (d) Schematic illustration of anatase/TiO2 -B microsphere and the additional Li+ storage at the interface; (e) cycling performance at the current densities in the range of 170 0–850 0 mA g−1 ; (f) cycling performance at 1700 mA g−1 with different contents of TiO2 -B, labeled as AB450 (14.96 %), AB550 (2.72 %), and AB650 (1.25 %). (d–f) Reprinted from Ref. [93].

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graphene, CNTs, metal nanoparticles, conductive polymers, etc. Moreover, the volume expansion can also be effectively buffered by the designed flexible/porous/hollow hybrid structures [89]. To date, considerable progress has been made on 2D hybrid metal oxides for batteries [2,56]. Here, two types of representative hybrids with promising applications for batteries are specifically emphasized. The first type is the layer-on-layer (2D-2D) configuration. This layer-on-layer design possesses obvious merits of excellent geometrical compatibility, strong adhesion, superior structural stability, and avoidable restack of nanosheets. For example, the layer-by-layer assembly of 2D Co3 O4 with atomic-level thickness and graphene (ATMCNs-GE) hybrid materials was achieved by controlling the ζ -potential of Co3 O4 and graphene (Fig. 4b) [90]. When evaluated for lithium ion storage, this composite could maintain over 90% of the initial specific capacity even after 20 0 0 charging/discharging cycles at the rate of 2.25 C (Fig. 4c) [90]. The second typical type of 2D hybrid metal oxides is 3D porous heterogeneous structures assembled from 2D constituent units. This structure can take full advantage of the merits of sheet-like structures and very attractive for ion storage due to improved active sites, increased transport channels, and enhanced buffering capacity for volume changes [91,92]. Wu et al. reported an interesting state of TiO2 microspheres composed of anatase/TiO2 -B ultrathin constituent nanosheets as electrode materials for LIBs [93]. This special heterojunction interfaces between the anatase and the Btype TiO2 in the nanosheets acted as diffusion channels and active sites for the transport and storage of Li+ (Fig. 4d). Electrochemical measurements revealed that the specific capacities of the as-prepared anatase/TiO2 -B hierarchical nanostructures were 180

(3400 mA g−1 ) and 110 mAh g−1 (8500 mA g−1 ) after 1000 cycles (Fig. 4e). It has also found that the optimized contents of TiO2 -B in the heterostructure were around 2.72%, which gave the highest capacity up to 10 0 0 cycles (Fig. 4f) [93].

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6. 2D metal oxide nanomaterials for other batteries

372

Other batteries, including metal-ion batteries (e.g. potassiumion battery, aluminum ion battery etc.), metal–air batteries (e.g. sodium–air battery, lithium–air battery, aluminum–air battery, etc.), metal–sulfur batteries (e.g. lithium–sulfur battery, magnesium–sulfur battery, zinc–sulfur battery, etc.), and some hybrid batteries (e.g. Zn–air/Ni batteries, etc.), have been paid more attention as alternative candidates to replace LIBs and NIBs as power sources. Fig. 5 shows the schematic illustration of Li–O2 batteries (Fig. 5a) [94], aluminum ion batteries (Fig. 5b) [95], potassium ion batteries (Fig. 5c) [96], and hybrid zinc–air/nickel battery (Fig. 5d–f) [97]. Among them, metal oxide nanosheets (e.g. MnO2 , Co3 O4 , etc.) have been reported as high-efficiency electrocatalysts in the oxygen electrode for Li–O2 /air batteries [94,98,99] and effective transfer mediator to inhibit polysulfide dissolution into electrolytes for Li–S batteries [45,46]. The related research on these post-lithium batteries is still in its initial stage and further indepth research is urgently required to fully understand the underlying reaction mechanisms of different batteries. In 2016, Lee et al. reported a novel Zn-air/Ni hybrid battery with NiO/Ni(OH)2 mesoporous spheres assembled from NiO/Ni(OH)2 nanoflakes as active material [97]. Compared with traditional hybrid battery systems where different battery reactions were simply

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Fig. 5. Schematic illustration of (a) Li-O2 battery. Reprinted from Ref. [94]; (b) super-valent battery based on the intercalation/deintercalation of Al ion. Reprinted from Ref. [95]; (c) potassium secondary battery. Reprinted from Ref. [96]; (d) hybrid zinc–air/nickel battery (left) and conventional zinc–air battery (right). Photography of (e) solid-state zinc–air/nickel hybrid battery prototype and (f) flexible hybrid battery demonstration. Reprinted from Ref. [97].

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connected via an external circuitry, this hybrid battery was operated based on two sets of fundamental reactions: the first set is the nickel reduction and oxidation reactions, and the second set is the oxygen reduction and evolution reactions (Fig. 5d). Based on this mechanism, a solid-state thin cell prototype with an overall thickness of only 0.5 mm has been manufactured (Fig. 5e) to evaluate its electrochemical performance. This battery could also be extended as a flexible hybrid cell for wearable devices (Fig. 5f). Owing to the combination of nickel–zinc and zinc–air reactions, both high energy density (980 Wh kg−1 ) and power density (gravimetric, 2700 W kg−1 ; volumetric, 14 000 W/L) were achieved for this hybrid battery. More attractive, this hybrid battery demonstrated a remarkable rate of charge, as high as ten times faster than the discharge rate but no obvious loss of capacity was found, making it very promising for large-scale practical applications [97].

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7. Conclusion and outlook

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In this article, three types of 2D metal oxide nanostructures, namely 2D binary metal oxide nanosheets, 2D ternary metal oxide nanosheets, and 2D hybrid metal oxide nanostructures, for the applications in high-performance batteries, have been summarized and reviewed. Generally, 2D ternary metal oxides consisting of two different metal cations and 2D metal oxide-based nanocomposites hybridized with some proper-complementary materials (e.g. carbon, CNTs, graphene, etc.) demonstrate better electrochemical properties than 2D binary metal oxides. Actually, the introduction of heterogeneous constituents can greatly enhance the entire conductivity of the materials. Most important, some unique synergic effects can generate to improve the rate capability and the cycling stability for the application as electrode materials for batteries. The controllable, cost-effective, and massive synthesis of the high-quality 2D metal oxide nanosheets as well as their composites, however, still remain as grand challenges, and further stud-

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ies on the underlying reaction mechanisms of these metal oxide nanostructures for ion transport and storage are also required. For the application in batteries, 2D metal oxide nanosheets are mainly investigated as electrode materials for LIBs. In spite of some explorations on SIBs, the electrochemical performances of those 2D nanomaterials are less satisfying. Meanwhile, the research of 2D metal oxides for the application in other batteries, such as postlithium batteries, is still at the infant stages or only the theoretical stage. In the practical applications, one common issue for 2D metal oxide nanosheets needs to be solved, which is the strong self-agglomeration tendency during the electrode assembly process. Hence, both the innovation of electrode assembly method and the design of novel 2D nanostructures are possible solutions to promote the future applications of 2D metal oxide nanosheets for batteries. Overall, 2D metal oxide nanomaterials have presented plenty of merits for the application in batteries. There is an unprecedented opportunity for the further development of metal oxide nanosheets for energy storage devices, even though some challenges exist. We believe that the rapid progress of 2D nanomaterials for electrochemical applications will significantly advance our knowledge of energy technology, nanoscience, and nanotechnology.

Uncited reference [88].

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Acknowledgments

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This work was supported by an Australian Research Council (ARC) Discovery Early Career Researcher Award (DECRA) project (DE150100280), an ARC Discovery Project (DP160102627), and an ARC Future Fellowship Project (FT160100281).

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